Earlier quoted context omitted.
He states 5-20 w/m2 (same as MW/km2) with a footnote that the theoretical one is 100 w/m2 but noone has achieved that yet.
https://en.wikipedia.org/wiki/Bhadla_Solar_Park This has a ~ 40 MW/ sq.km output with a lot of it still being open space for different reasons. https://en.wikipedia.org/wiki/Kamuthi_Solar_Power_Project This one has ~ 64 MW/ sq km output https://en.wikipedia.org/wiki/Copper_Mountain_Solar_Facility This one in US seems to be about ~ 53 MW/ sq km So definitely he is purposely understating the potential of solar. Because…
You are talking about peak power or capacity. He is talking about average, or generation. He explains the difference later in the chapter.